PROJECT SUMMARY Global transcriptional silencing is a highly conserved evolutionary event central to the transition from the fully differentiated oocyte to the totipotent embryo. Despite its importance in the development of all animals, this pivotal genome-wide event remains poorly understood. We have recently discovered that oocyte global transcriptional silencing depends on an mRNA decay activator. Oocyte-specific loss of ZFP36L2?an RNA-binding protein with a well-established role in AU-rich element-mediated mRNA decay?prevents oocytes from undergoing global transcriptional silencing. ZFP36L2-deficient oocytes are developmentally incompetent, with defects in maturation and fertilization leading to complete female infertility. Single cell RNA-seq analysis revealed that ZFP36L2 regulates scores of transcription regulators with central roles in chromatin modification and transcription initiation and elongation. This dysregulation resulted in failure to accumulate histone methylation marks associated with the silent, competent state. Our results define a critical role for an oocyte mRNA decay activator in the downregulation of transcription activators, leading to histone methylation, global transcriptional silencing and competence to transition from oocyte to embryo. These findings strongly point to a model in which global transcriptional silencing in the oocyte is mediated by mRNA decay. The goals of this proposal are to both uncover the mechanism(s) by which mRNA decay by ZFP36L2 brings about global transcriptional silencing in the oocyte and to take advantage of the insights provided by this unique genetic model system to investigate the role of ZFP36L2- dependent chromatin modifications critical for global transcriptional silencing and the successful transition from oocyte to embryo. To determine the role of mRNA decay in bringing about global transcriptional silencing, we will identify the direct targets of ZFP36L2 in the oocyte genome-wide and test the role of these factors in mediating global transcriptional silencing and oocyte developmental competence. We will determine the role of ZFP36L2 in chromatin modification, including histone H3 and DNA methylation, and test the role of these modifications in global transcriptional silencing in the oocyte. Finally, we will test whether ZFP36L2-dependent chromatin modifications established in the oocyte act to maintain global transcriptional silencing over the oocyte-to-embryo transition and/or set the stage for activation of new transcription in the newly formed embryo.